The antenna is a device which receives or transmits the front-end RF signal power in the form of an electromagnetic wave, and is an interface device between a circuit and the space for conversion between the guided wave and the free space wave energy. In a RFID system, the antenna is divided into two categories of an electronic tag antenna and a reader antenna, which are responsible for receiving and transmitting the energy, respectively. The current RFID systems mainly focus on the bands of low frequency (LF, 125 kHz-134 kHz), high frequency (HF, 13.56 MHz), ultrahigh frequency (UHF, 860-960 MHz) and microwave (MW, 2.45 GHz, 5.8 GHz). The principle and design for RFID system antenna in different operating frequency bands are fundamentally different. The gain and impedance characteristic of the RFID antenna may affect the operating distance of the RFID system, and the operating frequency band of the RFID system may in turn pose requirements regarding size and radiation loss of the antenna. Therefore, the design quality of the RFID antenna directly determines whether the overall RFID system is successful or not.
In a near-field antenna, as for the bands of low frequency (125 kHz-134 kHz) and high frequency (13.56 MHz), the system operates in near-field of the antenna. All of the energy required by the tag is obtained in an inductive coupling manner in the near field radiated by a coupling coil of the reader, and the operation manner is inductive coupling. In fact, the issue of propagation of electromagnetic wave is not involved in the near field, so that the design of antenna is relatively simple. Usually a coil antenna which is simple in process and low in cost is used. The coil antenna is indeed a resonant circuit. At the specified operating frequency, the coil antenna may produce resonance when the inductive impedance equals to the capacitive impedance.
In a far-field antenna, as for the bands of ultrahigh frequency (860 MHz-960 MHz) and microwave (2.45 GHz, 5.8 GHz), the reader antenna has to provide the tag with energy or wake up an active tag. The operation distance is relatively far, and generally is located in the far field of the reader antenna. According to the calculation equations of the far-field antenna, the electric field strength and the magnetic field intensity decay with the first power of the distance. The electric field and the magnetic field are orthogonal with each other in direction, and both fields are perpendicular to the propagation direction. The Poynting vector is a real number, and the electromagnetic field radiates energy in the form of electromagnetic wave. In this case, the design of antenna has a prominent effect on the performance of the system, and usually a dipole or microstrip patch antenna is used. The dipole antenna, also known as a symmetrical dipole antenna, is composed of two segments of straight wires which have the same thickness and length and are arranged in a straight line. A signal is fed in via two points in the middle, and a certain current distribution will be induced in two arms of the dipole. Such a current distribution will excite an electromagnetic field in the space around the antenna. Generally, a meander-line folded dipole antenna is used in the RFID electronic tag.
As for the existing RFID technology, since the applications are far less diversified and it is relatively difficult from the viewpoint of technology, the RFID chip usually has a unique operating frequency band, and only an antenna in a specific frequency band corresponds to the RFID chip. An attempt has been initiated in the field of mobile communication to develop the RFID to integrate the functions of communication, ID identification, and electronic payment. Therefore, the development of RFID technology in the future may exhibit the tendency of diversification in tag product, RFID multi-system integrated application, and the like. Thus, it is urgent to design and fabricate such a multi-system multi-band RFID antenna.